Phase shifting polygonal transformers and autotransformers are commonly used in three phase electrical networks supplying multiple non-linear loads. Non-linear loads, such as electronic equipment and equipment using various kinds of arc processes, produce undesirable harmonic currents within the network. Such harmonic-producing loads are becoming an increasingly large portion of the electrical load in many electrical networks, and can result in an unexpectedly high harmonic content in the electrical distribution system. This can lead to a number of problems which are well known to those skilled in the art.
Several techniques have been developed for reducing levels of different types of harmonics, including:
1. Different kinds of L-C filters tuned to different harmonic frequencies; PA1 2. Specialized filters such as zero phase sequence filters of various types for three phase, four wire systems with single phase loads; and PA1 3. Different kinds of phase shifters that allow for the creation of a quasi-multipulse system, and thus reduce harmonic levels for selected harmonics.
All of these techniques are well known and have been in widespread use for many years.
For three phase, three wire systems, polygonal phase shifting transformers and autotransformers are frequently used to reduce harmonics. However, in any case where more than two loads must be phase shifted, the use of conventional polygonal phase shifters has been problematic. It is difficult to keep the voltage levels of the various outputs within acceptable limits while obtaining the desired phase shift between outputs. This requirement considerably complicates the configuration of a conventional polygonal phase shifting transformer, as for example in the transformer described in U.S. Pat. No. 5,063,487 issued Nov. 5, 1991 for a Main and Auxiliary Transformer Rectifier System for Minimizing Line Harmonics.